This project deals with the interactions of vector-borne microorganisms, arboviruses, rickettsiae and protozoans, and cells of haematophagus arthropods, mainly ticks. Model in vitro systems are developed and applied to studies of pathogen host-range, growth dynamics, persistence and interference. After prolonged growth in these systems, pathogens are examined for changes in invasiveness, virulence, or antigenic characteristics. With overseas collaborators we are studying critical factors influencing survival of ticks and of pathogens within ticks from a biochemical, histochemical, and physiological standpoint. Two cell lines from Rhipicephalus ticks, which we recently established in continuous culture, were characterized with regard to karyology, growth characteristics and ability to support multiplication of viral and rickettsial agents. A study of the utilization of organic acids by tick cells in vitro was initiated. Serological characterization of two undescribed tickborne viruses has been completed.